LED lights require a DC voltage to operate. How you get the DC makes all the difference in the world. To better understand this, we need to have a basic understanding of what rectifier and diodes are. A rectifier is a device that simply converts alternating current (AC) into direct current (DC). Alternating current is current that flows one direction in a circuit on the first half of a sine wave and then the opposite direction on the second half. If we use a frequency of 60 Hz. (cycles per second) then the direction of current changes 120 times in one second. DC will always flow the same direction in a circuit. The way a rectifier changes AC to DC is by the use of a diode, or multiple diodes. Diodes only allow electrons to flow in one direction through them.
When the voltage on the anode is more positive than the voltage on the cathode, current will flow through the diode. If the voltage is reversed, making the cathode more positive, then current will not flow through the diode. (unless the peak reverse voltage rating is exceeded).
In the drawing below a single diode is used to rectify the alternating current. A half-wave DC output is produced. (click on drawing to view with animation)
On the first half of the cycle the diode allows the electrons to flow through it powering the DC load. On the second half cycle the diode blocks the flow of electrons and the load receives no flow at all. This is what causes the flicker effect of cheap LED lights.
When we use a four diode bridge rectifier , we solve the problem of having no flow to the load for half of the time. (click on drawing to view animation)
On the first half of our cycle the flow electrons passes through the bridge allowing electrons to flow to the load in one direction. Then on the second half of the cycle the flows passes through the bridge recifier following a different path, but still sends the electron flow in the same direction to the load giving it twice the output of a single diode rectifier. In the animation above, the cycles are moving at 1/8 cycle per second, while in actuality, this is happening 480 times faster at 60 cycles per second. An RC filter (a resistor and a capacitor wired in parallel with the DC load) further smooths the output reducing the flicker even more. So to answer our original question, LED Christmas lights that make use of full-wave bridge rectifier technology are worth spending a little more for.
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